Waveguide and method of manufacture

ABSTRACT

A hollow waveguide ( 1 ) has a wall ( 3 ) having plural pegs ( 16 ) thereon. The pegs ( 16 ) which project into the hollow interior of the waveguide ( 1 ) such that the waveguide ( 1 ) propagates electromagnetic waves only below a certain frequency. The surface of each of the pegs ( 16 ) is substantially free of discontinuity and concavities. The waveguide ( 1 ) may be manufactured by a moulding process.

The present invention relates to a hollow waveguide and to a method ofmanufacture of a waveguide.

Wireless communications offers many attractive features in comparisonwith wired communications. For example, a wireless system is very muchcheaper to install as no mechanical digging or laying of cables or wiresis required and user sites can be installed and de-installed veryquickly.

It is a feature of wireless systems when a large bandwidth (datatransfer rate) is required that, as the bandwidth that can be given toeach user increases, it is necessary for the bandwidth of the wirelesssignals to be similarly increased. Furthermore, the frequencies whichcan be used for wireless transmission are closely regulated and it is afact that only at microwave frequencies (i.e. in the gigahertz (GHz)region) or higher are such large bandwidths now available as the lowerradio frequencies have already been allocated.

A “mesh” communications system, which uses a multiplicity ofpoint-to-point wireless transmissions, can make more efficient use ofthe radio spectrum than a cellular system. An example of a meshcommunications system is disclosed in our International patentapplication WO-A-98/27694, the entire disclosure of which isincorporated herein by reference. In a typical implementation of a meshcommunications system, a plurality of nodes are interconnected using aplurality of point-to-point wireless links. Each node is typicallystationary or fixed and the node is likely to contain equipment that isused to connect a subscriber or user to the system. The nodes operate ina peer-to-peer manner, each node having apparatus for transmitting andfor receiving wireless signals over the plurality of point-to-pointwireless links and is arranged to relay data if data received by saidnode includes data for another node. At least some, more preferablymost, and in some cases all, nodes in the fully established mesh ofinterconnected nodes will be associated with a subscriber, which may bea natural person or an organisation such as a company, university, etc.Each subscriber node will typically act as the end point of a linkdedicated to that subscriber (i.e. as a source and as a sink of datatraffic) and also as an integral part of the distribution network forcarrying data intended for other nodes. The frequency used may be forexample at least about 1 GHz. A frequency greater than 2.4 GHz or 4 GHzmay be used. Indeed, a frequency of 28 GHz, 40 GHz, 60 GHz or even 200GHz may be used. Beyond radio frequencies, other yet higher frequenciessuch as of the order of 100,000 GHz (infra-red) could be used.

Within a mesh communications system, each node is connected to one ormore neighbouring nodes by a set of separate point-to-point wirelesstransmission links. When combined with the relay function in each node,it becomes possible to route information through the mesh by variousroutes. Information is transmitted around the system in a series of“hops” from node to node from the source node to the destination node.By suitable choice of node interconnections it is possible to configurethe mesh to provide multiple alternative routes, thus providing improvedavailability of service.

A mesh communications system can make more efficient use of the spectrumby directing the point-to-point wireless transmissions along the directline-of-sight between the nodes, for example by using highly directionalbeams. This use of spatially directed transmissions reduces the level ofunwanted transmissions in other spatial regions and also providessignificant directional gain such that the use of spatially directedtransmissions as a link between nodes allows the link to operate over alonger range than is possible with a less directional beam. By contrast,a cellular system is obliged to transmit over a wide spatial region inorder to support the point-to-multipoint transmissions. This istypically achieved in a cellular system by having a base station of thecellular system transmit a beam that has a very wide beam width inazimuth (typically being a sector of 60 degrees, 120 degrees oromnidirectional) but which is narrow in elevation, i.e. the beam from abase station in a cellular system is typically relatively horizontallyflat.

Because the preferred transmission frequency is in the microwave region,waveguides are used to couple the or each antenna with the associatedelectronics module that constitutes the transceiver electronics unit.

Waveguides typically comprise a conductive envelope which definesconditions that enable the propagation of electromagnetic waves. Typicalwaveguide configurations include those with a circular, a square orrectangular cross-section transverse to the direction of propagation.

Waveguides having a rectangular or square cross-section are a preferredmedium for propagation of waves in the microwave region and design toolsare available to enable the propagation characteristics of suchwaveguides to be set so as to constrain the propagation of waves alongthe waveguide. The fundamental mode of propagation in a rectangularwaveguide is the TE₁₀ mode. This fundamental mode has a single fieldmaximum across the width of the waveguide and no maximum along theheight direction of the waveguide.

To prevent the waveguide from propagating harmonics and other higherfrequencies, transverse slots in the form of corrugations across thewidth of the waveguide have been used to provide a low pass response tothe fundamental mode. However, such arrangements do not effectivelyattenuate higher order modes of the TE_(n0) type. Higher order modeshave two or more maxima across the width of the waveguide. So as tosuppress such higher order modes, longitudinal slots have been used.

One known type of filter which provides a low pass characteristic withhigh-order mode-suppression is the so-called “waffle-iron” filter. Suchwaffle-iron filters have arrays of identical opposed square pegsprojecting from the opposing broad walls of a rectangular waveguide. Thearrays of pegs of conventional waffle iron filters are created byconventional machining of two opposed walls which make up the waveguide.

An alternative arrangement is disclosed in JP-A-63/34408. This documentdiscloses a filter having cylindrical pegs which protrude from opposingwalls of a waveguide in which each of the pegs has a threaded end whichcooperates with a threaded aperture in the waveguide wall. Thisarrangement allows the propagation characteristic to be varied byscrewing the pegs into or out of the walls.

The difficulties of suitable alignment of opposing arrays of raised pegsand the problems of assembly of such devices were recognised in U.S.Pat. No. 3,777,286. This document discloses using die-casting techniquesto form generally square cross-section pegs, along with the wall fromwhich the pegs project and part of the side walls of the waveguide.

Where high frequencies are to be propagated, for example above about 10GHz, it has been believed that the small size of the componentsconcerned, and especially the precision required, necessitates precisionmachining or spark erosion on an internal surface of the waveguide wall.The physical dimensions of pegs in waveguide filters and similar devicesmust be tightly defined with stringent tolerances. This has theconsequence that conventional waveguide components are very expensive tomanufacture using these conventional techniques, which militates againsttheir use in consumer items.

According to a first aspect of the present invention, there is provideda hollow waveguide, the waveguide comprising a wall having plural pegsthereon which project into the hollow interior of the waveguide suchthat the waveguide propagates electromagnetic waves only below a certainfrequency, the surface of each of the pegs being substantially free ofdiscontinuity.

By forming pegs having surfaces that are substantially free ofdiscontinuity, the ability to mould or die-cast pegs in a consistentlyreproducible fashion is enhanced. It thus becomes possible to form thewaveguide by moulding, even though small dimensions may be used, thusallowing mass-production techniques to be used, thereby lowering themanufacturing cost dramatically (e.g. by a factor of 100 or so). Giventhat one principal intended application of such a waveguide is for usein nodes in a mesh communications system as described above rather thanfor example in one-off specialist applications, the ability to massproduce the waveguide at low cost is of paramount importance.

The surface of each the pegs is preferably substantially free ofconcavities. This further enhances the ability to mould the waveguide ina consistent and reproducible manner.

At least some of the pegs may have a substantially circularcross-sectional shape. Preferably, each peg has a substantially circularcross-sectional shape.

Alternatively or additionally, at least some of the pegs have asubstantially elliptical cross-sectional shape.

Other cross-sectional shapes are feasible.

At least some of the pegs preferably have a domed head. It has beenappreciated that the region on the pegs that is most liable tomalformation is the region nearest to the top of the pegs. The use of adomed head, which may for example be part spherical, avoids sharp edgesor other discontinuities which might otherwise affect the consistencywith which the waveguide can be formed.

At least one peg may have a convex fillet around its base at thejunction between the peg and the wall. This feature again helps to avoidsharp edges or other discontinuities which might otherwise affect theconsistency with which the waveguide can be formed.

The waveguide may comprise a second wall opposing the first wall andspaced therefrom, the face of the second wall that opposes the firstwall being substantially planar.

The waveguide may be dimensioned to propagate electromagnetic waveshaving a frequency of at least 10 GHz.

The waveguide may be dimensioned to propagate only electromagnetic waveshaving a frequency less than about 100 GHz.

According to a second aspect of the present invention there is provideda hollow waveguide, the waveguide comprising a wall having plural pegsthereon which project into the hollow interior of the waveguide suchthat the waveguide propagates electromagnetic waves only below a certainfrequency, each peg having a convex fillet around its base at thejunction between the peg and the wall.

According to a third aspect of the present invention there is provided ahollow waveguide, the waveguide comprising a wall having plural pegsthereon which project into the hollow interior of the waveguide suchthat the waveguide propagates electromagnetic waves only below a certainfrequency, each peg having a convex head.

According to another aspect of the present invention there is providedtransmitter-receiver apparatus, the apparatus comprising at least oneantenna for transmitting and receiving signals, an electronics modulefor providing signals to the antenna for transmission and for receivingsignals received by the antenna, and a hollow waveguide as describedabove selectively coupling the electronics module to the antenna.

According to another aspect of the present invention there is provided amethod of manufacture of a hollow waveguide, the waveguide comprising awall having plural pegs thereon which project into the hollow interiorof the waveguide such that the waveguide propagates electromagneticwaves only below a certain frequency, the surface of each of the pegsbeing substantially free of discontinuity, the waveguide being formedfrom a waveguide material, the method comprising: disposing a quantityof waveguide material into a mould tool having plural recesses in asurface therein, wherein each recess corresponds to a said peg; mouldingthe material; and, removing the hollow waveguide from the mould.

The waveguide material may comprise a plastics material. Said plasticsmaterial may be metallised plastics material.

Said moulding is preferably pressure die-casting.

According to another aspect of the present invention there is provided amethod of manufacture of a hollow waveguide, the waveguide comprising awall having plural pegs thereon which project into the hollow interiorof the waveguide such that the waveguide propagates electromagneticwaves only below a certain frequency, each peg having a convex filletaround its base at the junction between the peg and the wall, thewaveguide being formed from a waveguide material, the method comprising:disposing a quantity of waveguide material into a mould tool havingplural recesses in a surface therein, wherein each recess corresponds toa said peg; moulding the material; and, removing the hollow waveguidefrom the mould.

According to another aspect of the present invention there is provided amethod of manufacture of a hollow waveguide, the waveguide comprising awall having plural pegs thereon which project into the hollow interiorof the waveguide such that the waveguide propagates electromagneticwaves only below a certain frequency, each peg having a convex head, thewaveguide being formed from a waveguide material, the method comprising:disposing a quantity of waveguide material into a mould tool havingplural recesses in a surface therein, wherein each recess corresponds toa said peg; moulding the material; and, removing the hollow waveguidefrom the mould.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 shows a partial cut-away perspective view of a part of an exampleof a hollow waveguide in accordance with the present invention;

FIG. 2 shows a perspective view of examples of pegs having differentshapes;

FIG. 3 shows a schematic view of an example of a transmit-receive unitusing an embodiment of a hollow waveguide in accordance with anembodiment of the present invention;

FIG. 4 shows a cut-away view of the hollow waveguide of FIG. 3;

FIG. 5 shows a cross-section through the waveguide of FIG. 4 along theline V-V; and,

FIG. 6 is a longitudinally sectioned perspective view of an example oftransceiver apparatus suitable for use in a mesh communications system.

Referring first to FIG. 1, an example of a rectangular hollow waveguide1 has sixteen pegs 2 projecting from a base wall 3 into the hollowinterior of the waveguide 1. The waveguide 1 further has side walls 4and a top wall 5. Each peg 2 is of circular cross-section and has a sidewall portion 10 which extends from the base wall 3 of the waveguide 1via a convex bead or fillet 20. The side wall 10 of each peg 2 extendsto a domed head portion 11 of the peg 2. The pegs 2 are disposed in aregular array, the spacing A in the longitudinal direction beingselected to provide a low pass response and the spacing B in thetransverse direction being selected to suppress higher order propagationmodes.

The convex fillets 20 avoid a sharp transition at the base of the pegs 2between the peg 2 and the wall 3 of the waveguide 1. Such sharptransitions are difficult to mould and very difficult to mouldconsistently. The provision of an outwardly convex fillet 20 allows fora more easily reproducible shape at the base of the pegs 2 which leadsin turn to consistent behaviour between pegs 2 and between waveguides 1.The side walls 10 of the pegs 2 have a generally linear taper from thefillet 20 to a position 12 just under the domed head 11. The domed heads11 of the pegs 20 are substantially hemispherical in form. Thus, thecross-section of each peg 2 decreases generally linearly with distancefrom the wall 3 up to the position 12 and thereafter there is a rate ofdecrease of cross-section which increases with distance from the wall.The transition at the position 12 between the side wall 10 and the domedhead 11 is smooth, without sharp edges or other junctions. The use of adomed head 11 again avoids any sharp edges which, again, are difficultto mould and very difficult to mould consistently. For example, it hasbeen found that any attempt to mould say a flat head using mass-mouldingtechniques tends to produce a pyramid-like head owing to the smalldimensions that are required of the pegs when used in a waveguidetransmitting frequencies above 10 GHz. The shapes of such pyramids werefound to vary significantly between pegs 2 within a waveguide 1 andacross different waveguides 1. This is entirely avoided by use of a headwhich is free of discontinuities and particularly by use of a domed head11.

Moreover, the arrangement described above avoids any concavities in thesurface of the pegs 2, which again makes mass-moulding of the waveguide1 a realistic proposition even when the waveguide 1 is to be used forpropagation of high frequency waves.

Whilst a circular cross-sectional shape is preferred for the pegs 2,other cross-sectional shapes may be used. For example, thecross-sectional shape may be elliptical.

Referring now to FIG. 2, examples of alternative forms of peg are shown.A peg 30 is shown having a generally-tapering side wall 130 togetherwith a radiussed shoulder portion 230 leading to a flat top 330. In thiscase, the peg 30 is circular in cross-section with a radius r, theshoulder portion 230 having a radius of 0.2r. There is also shownanother peg 32 which has a side wall 131, a shoulder portion 231 havinga radius of 0.4r, and a flat top portion 331.

Referring now to FIG. 3, an example of a transmit-receive system 100comprises a transmit-receive antenna 101, such as a horn antenna, atransmit-receive electronics unit 102, and a waveguide module 105,including a matching filter 104 and a low-pass higher-order modesuppression filter 103, coupling the electronics unit 102 and theantenna 101. The low-pass higher-order mode suppression filter 103 isconstituted by a waveguide 1 of the type described above. Only one pathis shown between the electronics unit 102 and the antenna 101, but inpractice two or more separate paths may be provided.

The transmit-receive system 100 of the preferred embodiment is designedto operate at above about 10 GHz, and in a more preferred embodimentpropagates frequencies in the Ka band of between about 20 GHz and about40 GHz. In other embodiments, propagation at about 25 to about 30 GHz isenvisaged. A preferred operating frequency is about 28 GHz. The passcharacteristics of the waveguide 1 are preferably selected so as toreject frequencies of about 100 GHz upwards and more preferably toreject frequencies of about 50 to 60 GHz upwards, these being there-entrance modes at twice the operating frequencies.

The wavelength of a 10 GHz signal is 3 cm and the wavelength of a 28 GHzis just over 1 cm. It will be clear to those skilled in the art thatthese wavelengths determine the dimensions of the waveguide 1.

In one preferred embodiment, the width of the waveguide 1 is 7.11 mm,and the height is 3.56 mm. Pegs of the filter are 1.5 mm in height, havea base radius of 0.67 mm and are spaced by 2.66 mm in the longitudinaldirection and 2.66 mm in the transverse direction. In this embodiment,manufacturing tolerances are restricted to ±25 μm.

Referring now to FIG. 4, the waveguide module 105 has a first flange 200which has a rectangular opening for attachment to the antenna 101. Atthe end of an initial straight section, a first side wall 201 of thewaveguide module 105 is gently radiussed and passes through a rightangle and a second, opposed side wall 202 has a sharp radius and againpasses through a right angle so as to emerge parallel with the firstside wall 201. Passing along the waveguide module 105, a second straightsection of the waveguide module 105 has a step transformer or down-taper203 and then a filter 103 constituted by a waveguide 1 of the typedescribed above. In this example, the waveguide filter 103 has an arrayof fifteen pegs 204 projecting from a base wall 205. The array of pegs204 is in turn followed by a second step transformer or up-taper 206which leads to a second right angle bend. The second right angle bend,formed from a sharp turn in the first side wall 201 and a radiussed turnin the second side wall 202, leads to a third straight waveguidesection. The third straight section is parallel to the initial straightsection and has walls shaped to form an iris filter device 207,constituting a matching or decoupling filter 104. The filter 104 haseight opposed iris pairs 208A, 208B which project inwardly from the sidewalls 201,202 of the waveguide module 105. At the end of the thirdstraight section, the walls 201,202 lead to a further right angle bendto a fourth straight section parallel to the second section. This leadsvia a further right angle bend to a second flange 210 opening in thesame general direction as the first flange 200. The second flange 210 issecured to the transmit-receive electronics unit 102.

Referring now to FIG. 5, which is a cross-section on V-V of FIG. 4, thewaveguide module 105 has a top wall 220 which opposes the base wall 204.It will be seen that the step transformer or down-taper 203 reduces theheight of the waveguide module 105 substantially in the region of thefilter 103 so that the tops of the pegs 204 of the filter 103 arerelatively close to the top wall 220. The following up-taper or steptransformer 206 restores the height of the waveguide module 105.Although the embodiment shown in FIGS. 4 and 5 has only a single arrayof pegs 204 co-operating with a planar top wall 220, it will beunderstood that two opposed sets of pegs could be provided instead withone set being on the base wall 204 and the other on the top wall 220.

FIG. 6 shows an example of transceiver apparatus 300 suitable for use ina mesh communications system as described above. A generally columnarsupport structure 301 supports four antennas 101. This support structure301 is more fully described in our WO-A-02/50950, the entire disclosureof which is hereby incorporated by reference. Each antenna 101 of thisexample is suitable for the transmission and reception of radio orhigher frequencies, typically at 1 GHz or higher frequencies, such as2.4 GHz, 4 GHz, 28 GHz, 40 GHz, 60 GHz or even 200 GHz; beyond radiofrequencies, other yet higher frequencies such as of the order of100,000 GHz (infra-red) could be used. In use, the support structure 301will normally be orientated vertically so that its central longitudinalaxis is vertical and each antenna 101 is therefore normally arranged totransmit and receive in a direction that is substantially centred inelevation on the horizontal plane, i.e. typically within about ±5° ofthe horizontal plane.

Each antenna 101 is mounted in its own antenna support 302. In theexample shown, there are four antenna supports 302 each for supporting arespective antenna 101. For economy of manufacture, it is preferred thatall antenna supports 302 be substantially identical (i.e.constructionally and/or functionally the same as each other except forminor or inconsequential differences, including those that might arisethrough variations in the manufacturing process). Each antenna support302 of this example is generally in the form of a hollow cylinder ofcircular cross-section. Each antenna support 302 is able to rotate aboutan axis of rotation which in use is vertical. The cylindrical side wallof each antenna support 302 is recessed on one side to receive anantenna 101 and is provided with screw fixing holes which can receivescrews for fixing the antenna 101 to the antenna support 302. In thisexample, an external radome 303 surrounds the antenna supports 302.

Neighbouring antenna supports 302 are connected together via a bearing304 which is provided at the junction between the neighbouring antennasupports 302 and which allows the neighbouring antenna supports 302 torotate relative to each other.

In the example shown in FIG. 6, a single transceiver unit 102 iscontained in every other antenna support 302. Typically, the transceiverunits 102 will be radio modules. The transceiver units 102 contain allof the necessary circuitry to allow signals to be transmitted andreceived via the antennas 102. Each transceiver unit 102 services theantenna 101 provided in the same antenna support 302 as well as theantenna 101 provided in a neighbouring antenna support 302 (in theexample shown, the lower neighbouring antenna support 302). In theexample shown in which the wireless transmissions to and from theantennas 101 are at microwave frequencies (approximately 1 GHz orhigher), waveguides 105, preferably as described above, are provided toconnect the radio module 102 to the respective antennas 101. Thepreferred waveguides 105 provide a low pass frequency filtering functionand act to suppress higher mode propagation, as discussed in more detailabove.

The manufacture of a hollow waveguide in accordance with the preferredembodiment of the present invention follows conventional moulding ordie-casting techniques. That is, a mould is provided and mouldingmaterial is applied to the mould, preferably under pressure, to form thewaveguide. The shape of the moulding tool is designed to allow releaseof the product from the tool by virtue of the previously-discussedshapes.

The moulding or die-casting material may be a metal, or a metal alloy.It is also possible to form the device by metallised plastics moulding,i.e. by moulding the waveguide in plastics and then coating thewaveguide with metal.

Embodiments of the present invention have been described with particularreference to the examples illustrated. However, it will be appreciatedthat variations and modifications may be made to the examples describedwithin the scope of the present invention.

1. A hollow waveguide, the waveguide comprising a wall having plural pegs thereon which project into the hollow interior of the waveguide such that the waveguide propagates electromagnetic waves only below a certain frequency, the surface of each of the pegs being substantially free of discontinuity.
 2. A waveguide according to claim 1, wherein the surface of each the pegs is substantially free of concavities.
 3. A waveguide according to claim 1, wherein at least some of the pegs have a substantially circular cross-sectional shape.
 4. A waveguide according to claim 3, wherein each peg has a substantially circular cross-sectional shape.
 5. A waveguide according to claim 1, wherein at least some of the pegs have a substantially elliptical cross-sectional shape.
 6. A waveguide according to claim 1, wherein at least some of the pegs have a domed head.
 7. A waveguide according to claim 1, wherein at least one peg has a convex fillet around its base at the junction between the peg and the wall.
 8. A waveguide according to claim 1, comprising a second wall opposing the first wall and spaced therefrom, the face of the second wall that opposes the first wall being substantially planar.
 9. A waveguide according to claim 1, wherein the waveguide is dimensioned to propagate electromagnetic waves having a frequency of at least 10 GHz.
 10. A waveguide according to claim 1, wherein the waveguide is dimensioned to propagate only electromagnetic waves having a frequency less than about 100 GHz.
 11. A hollow waveguide, the waveguide comprising a wall having plural pegs thereon which project into the hollow interior of the waveguide such that the waveguide propagates electromagnetic waves only below a certain frequency, each peg having a convex fillet around its base at the junction between the peg and the wall.
 12. A hollow waveguide, the waveguide comprising a wall having plural pegs thereon which project into the hollow interior of the waveguide such that the waveguide propagates electromagnetic waves only below a certain frequency, each peg having a convex head.
 13. Transmitter-receiver apparatus, the apparatus comprising at least one antenna for transmitting and receiving signals, an electronics module for providing signals to the antenna for transmission and for receiving signals received by the antenna, and a hollow waveguide according to claim 1, selectively coupling the electronics module to the antenna.
 14. A method of manufacture of a hollow waveguide, the waveguide comprising a wall having plural pegs thereon which project into the hollow interior of the waveguide such that the waveguide propagates electromagnetic waves only below a certain frequency, the surface of each of the pegs being substantially free of discontinuity, the waveguide being formed from a waveguide material, the method comprising: disposing a quantity of waveguide material into a mould tool having plural recesses in a surface therein, wherein each recess corresponds to a said peg; moulding the material; and, removing the hollow waveguide from the mould.
 15. A method according to claim 14, wherein the waveguide material comprises a plastics material.
 16. A method according to claim 15, wherein said plastics material is metallised plastics material.
 17. A method according to claim 14, wherein said moulding is pressure die-casting.
 18. A method of manufacture of a hollow waveguide, the waveguide comprising a wall having plural pegs thereon which project into the hollow interior of the waveguide such that the waveguide propagates electromagnetic waves only below a certain frequency, each peg having a convex fillet around its base at the junction between the peg and the wall, the waveguide being formed from a waveguide material, the method comprising: disposing a quantity of waveguide material into a mould tool having plural recesses in a surface therein, wherein each recess corresponds to a said peg; moulding the material; and, removing the hollow waveguide from the mould.
 19. A method of manufacture of a hollow waveguide, the waveguide comprising a wall having plural pegs thereon which project into the hollow interior of the waveguide such that the waveguide propagates electromagnetic waves only below a certain frequency, each peg having a convex head, the waveguide being formed from a waveguide material, the method comprising: disposing a quantity of waveguide material into a mould tool having plural recesses in a surface therein, wherein each recess corresponds to a said peg; moulding the material; and, removing the hollow waveguide from the mould. 